JavaScript: Better way to sort an array by setting custom index

Question

I'm trying to create a sorted array based on the below input data:

Input:

[{"title": "100_baz"}, {"title": "01_foo"}, {"title": "05_bar"}]

function indexSort (input) {    
    let output = [];
    input.forEach(value => {    
        if (value.title && value.title.indexOf('_') !== -1) {
            let targetIndex = parseInt (value.title.split('_')[0]);
            let title = value.title.substring(value.title.indexOf('_') + 1, value.title.length); // extract the string next to '_'
            output[targetIndex] = title;
        }    
    })
    console.log(output)
}

Desired Output:

I'm able to get the desired output using the above code snippet, however, I'm looking for a better way to achieve this in terms of good practice, or a concise and clear method in ES5 or ES6.

Answer 1

Updating my comments on current implementation, Thanks @Mast for pointing out.

1. You have used let for output, targetIndex and title. None of them never updated. const is better suitable here.
2. Usage of value.title.indexOf('_') is repeated. you can use variable.
3. You can avoid substring usage because you are already did value.title.split('_') and you can use index 1 value from result array.

Apart from these minor points, code looks good to me.

As you asked, Here is alternative ES6 implementation.

1. Use arrow function
2. With reduce and using destructure in input function arguments and have the default value for title.
3. In reduce function use destucture to get the values from split result.

const data = [{"title": "100_baz"}, {"title": "01_foo"}, {"title": "05_bar"}];

const indexSort = data => data.reduce((acc, {title = ''}) => {
  const [num, value] = title.split('_');
  acc[Number(num)] = value;
  return acc;
}, []);

console.log(indexSort(data));

Answer 2

Your code is pretty sound. I can't see any improvements that specifically use ES5/ES6 tricks - the spread operator for example will set all of the other keys to undefined instead of leaving them empty, which I think is undesired. Further loops would iterate over those undefined keys as well.

That said, here are some improvements:

• Use reduce to remove the need for a new array and replace the forEach
• Store the return value from split and use that to replace the indexOf (if the delimiter doesn't exist split returns an array of length 1) and substring calls
• At the moment, the indexSort function isn't as single-purpose as it could be. It has to know that the input array contains a title key. This can be moved outside to where the function is called making indexSort reusable
• The name indexSort is a little misleading - I assumed it was going to sort a preexisting array. Perhaps use something like arrayFromTitles instead?

Quick snippet showing these changes:

function indexSort (input) {
    return input.reduce((accumulator, current) => {
        let split = current.split('_');
        if ( split.length > 1 ) {
            accumulator[parseInt(split[0])] = split[1];
        }
        return accumulator;
    }, []);
}

let titles = [{"title": "100_baz"}, {"title": "01_foo"}, {"title": "05_bar"}];
indexSort(titles.map(x => x.title));